The present invention relates to magnets and similar means for producing highly uniform, high strength magnetic fields. More particularly, the present invention is related to the construction of magnets for producing high homogeneity magnetic fields for use in nuclear magnetic resonance (NMR) imaging systems.
In virtually all NMR applications, it is necessary to provide a spacially constant magnetic field, B.sub.o. It is also desirable that the magnetic field comprise an essentially axial component, so that, ideally B.sub.z =B.sub.o, where B.sub.z is the component of the magnetic field in the z-axis direction of a conventional cylindrical coordinate system. Likewise, in the ideal situation, it is desired that the radial component B.sub.p =O in the volume of interest. The magnetic field is important since it establishes nuclear spin precession distribution in the subject volume. Subsequent radio frequency radiation from excited nuclei in this volume are employed, under appropriate circumstances, to produce imaging data and spectroscopic chemical information corresponding to physical objects and processes placed within the volume of interest. It has also been recently seen that NMR imaging is particularly applicable to the generation of medical diagnostic images. However, in such applications it has been found that it is particularly desirable to employ a highly uniform magnetic field. It has also been found that the use of superconductive materials for the construction of high strength, high homogeneity magnets is particularly advantageous. In particular, superconductive properties may be exploited to provide magnet coils carrying high, persistent currents, often at a level of between about 1,000 and 2,000 amperes. The use of superconductive materials has permitted the construction of a magnet for NMR imaging which produces a magnetic field of approximately 1.5 Tesla (15,000 gauss). These high field strengths are particularly advantageous in that the signal to noise ratio in the NMR imaging system is improved. Furthermore, it is seen that superconductive magnets do not require the supply of constant power as do conventional resistive magnets. Furthermore, the important properties of field homogeneity and stability can be better maintained with superconductive magnets than with resistively excited magnet devices. Accordingly, it is seen that it is highly desirable to be able to produce high strength, high uniformity magnetic fields.
At present, superconductive magnets employed for NMR imaging also typically employ a set of correction coils for the purpose of achieving greater magnetic field uniformity. One or more sets of these correction coils may be required. The field contribution of a correction coil is designed to be nonuniform, so that in combination with the main magnetic field, the field of the correction coil acts to reduce overall magnetic field non-uniformity. Correction coils are typically designed to carry only a small fraction of the current carried by the main superconductive coils. This is a natural consequence of the fact that the correction coils are only designed for producing small correction fields to counter-balance main coil field non-uniformity. In systems where both the main coils and the correction coils are superconducting, it is sometimes the practice to equip both the main coils and the correction coils with persistent current switches, which are devices which short circuit the coils with an element which is superconductive or resistive at the operator's discretion. When currents in the main coil and all the correction coils have been adjusted to satisfactory levels, the switches can be changed to the superconductive state (persistent mode), and the system of coils thereafter preserves its adjustment until it is intentionally disturbed. It should be kept in mind that high field uniformity is an important aspect of the present invention.